Asian grocers fresh or frozen; supermarkets sell it in tubes as paste.
1Frozen lemongrass
The same
You lose very little.
2Lemongrass paste
About a tablespoon per stalk
You lose some freshness; tubes are often sweetened or salted, so taste first.
3Lemon zest
The zest of half a lemon per stalk
You lose the grassy, gingery side.
Gets some back a slice or two of ginger.